Another chapter in the drive train story


#1

Now, in all the threads that have been discussed rescently, no one has mentioned a thing about weight distrubution…

So we know how to figure out optimum power and what type of material works well for wheels and treads, but we haven’t discussed why these different materials have worked well for certain robots.

The difference that has to be considered is how the weight is distributed among the drive wheels/tread. What was the positioning of the drive train of everyone’s robots? How did you go about keeping a low CG and still keep the weight in the optimal position to help the drive train? Was this solved by four wheel drive or by other means?

More preseason thinking…


#2

Our gearbox was at the “rear” of the robot, however the robot can be driven with either end the front. It drove the “rear” axles and sprockets and chains conected the front and rear axles for 4 wheel drive. Everything else on the robot was very semetrical and balanced so to counteract the rear gearbox, the battery was placed more near the front (opposite end). Everything is kept relatively low as our robot is only a total of about 14 inches tall.


#3

we mounted our battery on the bottom of the robot to help keep a low CG. we also put our air storage tanks down there. plus 5 motors, gears, sprokets, and chains. we had 2 drive wheels in the middle and 4 castors slightly raised at the ends. this kept most of the weight on our drive wheels.


#4

Our robot was pretty low to the ground (only 20-some inches tall) and everything but the grabbers sat on the main deck inside. We put the compressor in the front right over the wheel and the battery in the back left over a wheel. The arms went down the middle, but everything else was on a plywood sheet at the mid-wheel level. Our center of gravity was VERY low, unlike last year’s bot (which I won’t get into… 13’ arms, 26" wheelbase…ugh)…


#5

13 foot arms:eek: :eek: :eek:

Pictures please

pictures pictures pictures pictures pictures


#6

Well, the arms are not quite 13 feet. The robot with arms extended straight up is 13 feet tall. The arms themselves are 8 feet, the robot is 5, 8+5=13. So I cheated a bit…

So, without further adue… I present Tippy, our stability-challenged, much-cursed 2001 bot:

p1010031.jpg


p1010031.jpg


#7

Thanks for the great picture. I joined the FIRST program for the 2002 season so I hadn’t really seen those giant lifting arms before.

I have a question. If you look at the robot in the left of the picture, the wheels are on the outside of the frame. I’ve seen a few battlebots like this too. Is there any advantage to having the wheels on the outside?


#8

I forgot to mention a couple things.

First of all, I think all the threads involved with “the drivetrain story” are GRRREAT. Kudos to Patrick Wang for getting us going on something important. I was even inspired enough to go buy a CAD program.

Anyway, what I really posted for is I was wondering how many robots use treads instead of wheels? Does it give that much more traction? I would personally use them for the coolness factor alone but the pulleys are between $80 and $120 apiece!!! How hard would it be to make our own. Could it be done on manual mills/lathes or would we need CNC.


#9

*Originally posted by sanddrag *
**Thanks for the great picture. I joined the FIRST program for the 2002 season so I hadn’t really seen those giant lifting arms before.

I have a question. If you look at the robot in the left of the picture, the wheels are on the outside of the frame. I’ve seen a few battlebots like this too. Is there any advantage to having the wheels on the outside? **

As the Pontiac commercial people will tell you, Wider is Better. To see the effect on stability, take it to an extreme, and imagine placing the wheels very close to each other in the middle, or on the very far edges far apart from each other.


#10

Our robot this year could pull a lot. (don’t remember how much though) We had 4 wheels with this kind of elevator belt that has a lot of grip. Personally, tracks weigh more and are harder and more $$ to make. But if you can have the weight, $$, and time then sure, use tracks.

PS i included a pic of our fancy wheels w/ the tread.

robot6bb.jpg


robot6bb.jpg


#11

*Originally posted by sanddrag *
**I forgot to mention a couple things.

First of all, I think all the threads involved with “the drivetrain story” are GRRREAT. Kudos to Patrick Wang for getting us going on something important. I was even inspired enough to go buy a CAD program.

Anyway, what I really posted for is I was wondering how many robots use treads instead of wheels? Does it give that much more traction? I would personally use them for the coolness factor alone but the pulleys are between $80 and $120 apiece!!! How hard would it be to make our own. Could it be done on manual mills/lathes or would we need CNC. **

What CAD program did you get? Also, while an NC machine isn’t required to do belt pulleys, it would save time and effort. You could use a thing called a dividing head on a manual mill. It could also be used to make gears.


#12

The CAD program I bought is made by Swift software and is called 3D CAD. It was $2.88 on the clearance table at Office Depot. The program is worth nothing for scale drawings or precise measurements but it works excellently for laying things out for initial concepts. It is also very useful for showing other people your ideas. For $3, it can’t be beat.


#13

most of our bots from the last couple of years were EXTREMELY LOW to the ground!! 2001 we had 3inches of ground clearence, this year we had 3/8ths of an inche!!! we use wheels that we make ourselfs, chassis are low to the ground and not alot about the cg.

pics:

good wheel1.jpg


good wheel1.jpg


#14

Our robot had an intriguing design. To ease zero radius turns, we had only two drive-wheels mounted in the center of the robot and had castors on the front and back. Most of the weigh was centered low around the chasis plate and the battery was the lowest point on our robot (beside the wheels) which lowered the center of gravity substantially. The wheels were powered by two drill motors and a 200:1 gear / chain reduction. Thus, to keep the weight on the wheels, we just balanced the wheels. However, I do suspect that if we had used more drive wheels, we could have gotten more traction, which always helps, but we wanted to tread lightly on the carpet.


#15

We’ve used the two-wheel drive with castors setup before. It’s worked pretty well for us. It turns well, and can push okay, but sometimes you just need the power and traction 4-wheel-drive offers. Plus, if there is any kind of incline, that setup won’t work. You’ll high-center your 'bot when you try to go up the ramp. It’s not perfect, but it’s surely simple and effective.


#16

Our robot was unique this year because it was nothing but a drivetrain. Obviously, that helped us keep a low center of gravity.

Our ground clearance was approximately 1/8" - despite that, another robot managed to lift us off our wheels at Nationals :frowning:

The motors were mounted roughly midway between the four drive wheels. Our treads occupied the entire footprint of the robot, practically. They were all driven by a common axle. The battery was actually the highest part of our robot, directly above the motors. It was, however, only 15" or so off the floor.


#17

*Originally posted by Michael Krass *
** Our ground clearance was approximately 1/8" - despite that, another robot managed to lift us off our wheels at Nationals :frowning:
**

Yeah, our clearance was was ~ 1/2" and we managed to ride up on a row of balls on the Einstein field during one of our elim. matches at Nationals this year … at one point during the match, we only had one wheel touching the ground! :frowning:

  • Katie

#18

We had a lot of ground clearance with our 10" wheels which was probably a mistake. When I was driving I rode up completely on top of the soccer balls. If I didn’t throw it in reverse, the robot would have gone over the side guard rail of the field. (that would have been sooo funny :smiley: lol)


#19

Frostbite II
specs 95 lbs
2 drill
2 chipua
4 custom gear boxes only 1" wide, fixed speed
2.5 mph
tank drive
4 6" sold aluminum wheels
bumpers…ha… a protective rim of 28mm extrusion
2" ground clearance

We were never pushed, this was our second atempt at a four wheel drive or, the second in the frostbite series.

Frostbite I
specs 65 lbs
2 drill
2 fisher price
Custom gearboxes for the drills
Tanks drive
4 specially milled nylon wheels
2.5 mph
28mm extrusion bumper protection
2" ground clearance

the drive trains of the Penguins


#20

Weight distribution is very important for the wheels to have any grab. Trying to concentrate robot weight over the driven wheels will insure you get maximum grip for the matierial you use. We have used a special belting from Small Parts that is brown and has a lot of teeth for gripping carpet. As with any matierial, any drive sytem that causes the wheels to slip on the carpet will eventually wear the surface of the drive wheels. So was the case of the brown belting we used.
As to robot design, I saw many robots in the past that had little ground clearance and a robot frame that extended well outside the driven wheels. Whenever the robot tipped a little, the frame would contact the floor and the wheels would be lifted off the carpet. In this case it doesn’t matter very much where the weight is, no contact means no drive power.
Low center of gravity is a must to keep all the wheels on the floor and maximize stability and drive power transfer.